1. Field of the Invention
The present invention relates to an optical scanning lens and apparatus, and more particularly to an optical scanning lens and apparatus which are capable of effectively generating accurately-pitched light spots on a latent image recording surface. Further, the present invention also relates to an image forming apparatus using the above-mentioned optical scanning apparatus.
2. Discussion of the Background
Image forming apparatuses such as digital copying machines, laser printers, and laser facsimile machines form images generally by using optical scanning systems. Such an optical scanning system includes a light source having a light emission point, a first optical mechanism, a light deflecting mechanism, and a second optical mechanism. The first optical mechanism reforms the shape of the light beam emitted from the light emission point so that the light beam forms a line image extending in the main scanning direction at an area to the light deflecting mechanism. The light deflecting mechanism deflects the light beam to convert the light beam into a scanning light beam with a plurality of deflective reflection surfaces. The second optical mechanism focuses the scanning light beam into a scanning light spot on a surface to be scanned.
A multi-beam scanning optical system, also known, is capable of scanning simultaneously with multiple laser beams by using a light source that has a plurality of light emission points.
In recent years, optical elements for use in an optical scanning apparatus, such as an optical scanning lens in particular, are made of molded plastic due to a reason that a lens with a high quality non-circular surface is produced by plastic molding at a relatively low cost. In a process of plastic molding for optical elements, a plastic material melted by heat is molded with a molding tool and is cooled in the molded tool. At this time, however, a cooling speed of the plastic material is different between peripheral portions and the center portion of the plastic material. That is, the peripheral portions of the plastic material are cooled faster than the center portion thereof. As a result, the peripheral portions have a higher density than the center portion. This causes an uneven density distribution in the plastic lense, or a deformation of the lens which produces art, uneven refractive index in the plastic lens. As a consequence, a refractive index profile is generated in the plastic lens.
A refractive index profile of a background lens 50 is explained with reference to FIGS. 1A-1E. FIG. 1A is a cross-section view of the lens 50 with contour lines expressing a refractive index profile of the lens 50 seen from this view. This cross-section view is made by virtually cutting the lens 50 with a plane including a light axis and parallel to the main scanning direction. In FIG. 1A, a dotted line represents a thickness center line connecting centers of lens thickness along in the main scanning direction. FIG. 1B shows the refractive index profile of the lens 50 along the thickness center line of the lens 50 indicated by the dotted line in FIG. 1A.
FIG. 1C shows a cross-section view of the lens 50 with contour lines expressing the refractive index profile of the lens 50 seen from this view. This cross-section view is made by virtually cutting the lens 50 with a plane including the light axis and parallel to the sub-scanning direction. FIG. 1D shows the refractive index profile of the lens 50 along a thickness center line of the lens 50 of FIG. 1C. FIG. 1E shows the refractive index profile of the lens 50 along the plane including the light axis and parallel to the main scanning direction.
As shown in FIGS. 1A-1E, the refractive index inside a lens is generally higher at peripheral portions of the lens than at the center portion thereof. This is caused by the different cooling speed in the plastic molding, as described above.
When a lens for use in an optical scanning system has a refractive index profile, optical characteristics of the lens become slightly different from those as designed. This is because when the lens is designed, the refractive index inside the lens is considered to be consistent. Accordingly, an average lens has the refractive index higher at the peripheral portions thereof than at the center portion, and focuses light into a light spot at a position on a scanning surface that is slightly longer in distance from a light deflecting than the position as designed.
A diameter of the light spot running within an effective scanning area on the scanning surface, as well as an image height, vary in accordance with a curve in an image surface of the optical scanning lens. When the lens has the refractive index profile, the diameter of the light spot is also effected. FIG. 2 shows a manner in which an amount of defocus on the scanning surface is changed by the refractive index profile. In FIG. 2, the vertical axis represents a diameter of light spot and the horizontal axis represents a defocus amount, that is, a difference between an image focusing position (i.e., the light gathering position) and a position on the scanning surface. When a lens has a consistent refractive index inside the lens and has no refractive index profile, a relationship between the defocus amount and the spot diameter is made as indicated by a dotted line A in FIG. 2 in which the spot diameter becomes smaller *** on the scanning surface, that is, at a position where the defocus amount is 0. When a lens has a refractive index profile, the relationship between the defocus amount and the spot diameter is moved rightwards as indicated by a solid line B in FIG. 2. In this case, the spot diameter on the scanning surface becomes greater than the diameter as designed. Accordingly, the actual spot diameter on the scanning surface is greater than the designed spot diameter by an amount indicated by a letter C.
Moreover, a deviation of focusing position caused due to the refractive index profile is not necessarily consistent among image heights. If the deviation of focusing position is consistent among image heights, a good light spot can be made to every image height by moving a portion of the first optical mechanism in the light axis direction to correct the focusing position towards the dotted line A.
However, when a lens having a refractive index profile is used, the deviation of focusing position is not consistent among image heights. Accordingly, an adjustment of a good light spot taken for an image height may not be applicable to another image height. This becomes apparent as the light spot diameter is made small in order to increase an image quality.
Therefore, when the refractive index profile is not taken into account in designing a lens for use in an optical scanning system, the variations of the light spot diameter among the image heights become greater and degrade an image recording quality.
If a lens having the refractive index profile is used in a multi-beam scanning optical system, a pitch of the multiple beams on the scanning surface may vary among the image heights. The reason is that a horizontal scaling and a refractive direction of the image focusing in the sub-scanning direction from the light deflecting mechanism to the scanning surface are changed over each image height. When a deviation of the light spot diameter among the image heights is greater, an uneven recording image is produced. This phenomenon may become apparent as the pitch of the light spots is adjusted narrower to increase an image quality. Examples of the optical scanning apparatus are described in Japanese Laid-Open Patent Application Publications, No. 09-049976, No. 10-288749, No. 11-002768, No. 11-038314, and No. 11-04461.
The optical scanning apparatus described in Japanese Laid-Open Patent Application Publication, No. 09-049976, is configured to shorten a focal length calculated based on a curvature of a lens surface, a refractive index of a lens material, and a light-axis thickness of the lens in comparison with an actually measured focal length. This is one way to attempt to correct a deviation of image focusing position due to the refractive index profile generated through the plastic molding process.
Since the molded-plastic lens is usually produced through a mass production process, as described above, using the same material and under the same conditions, the refractive index profile is consistent among the produced lenses. Therefore, the refractive index profile of the lens can be experimentally measured. For example, a refractive index profile can be determined within an effective diameter of the lens using a formula V≦15×10−5, wherein V is a difference between the largest and smallest refractive indexes measured, representing an amount of the refractive index profile. Therefore, after the molding process with the molding tool, the refractive index profile is measured and is used to correct the shape of the molding tool. In this way, the deviation of the image focusing position caused by the refractive index profile can be corrected.
In correcting the shape of the plastic molding tool, an amount of correction is as preferably small as possible since a smaller correction can ensure an easy and accurate correction. However, as described in Japanese Laid-Open Patent Application Publication, No. 09-049976, an amount of correction with respect to the shape of the plastic molding tool is considerably large when the deviated image focusing position due to the refractive index profile is correctively shifted to the scanning surface by determining the curvature and associated factors of the lens such that the focal length is made shorter than the actually measured focal length over every image height. In this case, it is difficult to perform the correction with accuracy. If the deviation of the image focusing position is corrected, a value of F/W may be of the order of 0.007, wherein W is an effective recording width on the scanning surface for the light spot to run and F is an amount of deviation among the image heights with respect to the image focusing position of the light spot. However, to decrease the light spot diameter to increase an image quality, the value of F/W is needed to be made smaller than the above-mentioned value.
As described above, when a lens has the refractive index profile and when the deviation of focusing-position is consistent among image heights, a good light spot can be made to every image height by moving a portion of the first optical mechanism in the light axis direction. Accordingly, correction of the shape of the plastic molding tool may be changed. That is, the correction amount is not determined in a way such that the deviated image focusing positions are adjusted relative to the scanning surface over every image height. Instead, the shape of the plastic molding tool is determined so as to make the deviation amount among the image heights smaller. Thereby, the shape of the plastic molding tool can be corrected in a superior manner with a minimum correction amount even though the deviation of the image focusing position from the scanning surface remains.
The optical scanning apparatus described in Japanese Laid-Open Patent Application Publication, No. 10-288749, uses an optical scanning lens having a sufficient margin of focal depth to attempt to generate a good light spot even if the lens has the refractive index profile However, when the diameter of the of the light spot is decreased, it becomes difficult to maintain the sufficient margin of focal depth so that errors in manufacturing and assembling with respect to the lens are severely eliminated. This leads to an increase of coat, while the attempt is not preferable from the viewpoint of image quality.
The optical scanning apparatus described in Japanese Laid-Open Patent Application Publication, No. 11-002768, includes the first optical mechanism which is configured to attempt to correct the deviation of the image focusing position. This apparatus cannot correct the image focusing position over each image height, while the first optical mechanism can correct the deviation of the image focusing position among the image heights by the same amount in the same direction. This apparatus may be effective in a case where the refractive index profile is extremely small and an amount of the deviation of the image focusing position is constant. However, the deviation of the image focusing position caused by the refractive index profile of the actual lens is different among the image heights and such deviation becomes apparent as the lens has a greater amount of deviation of the lens thickness. Therefore, under such conditions, this apparatus cannot generate a good light spot.
The optical scanning apparatus described in Japanese Laid-Open Patent Application Publication, No. 11-038314, is configured to attempt to correct the deviation of the image focusing position caused by the refractive index profile towards the minus side from the scanning surface in the middle area of the image height and towards the plus side from the scanning surface in the peripheral area of the image height. As in the case of Japanese Laid-Open patent Application Publication, No. 11-002768, the apparatus may be effective in a case where the refractive index profile is extremely small and an amount of the deviation of the image focusing position is constant. However, a good light spot cannot be generated with this apparatus.
In addition, none of the apparatuses described by the above mentioned publications describes the pitch of the light spots and a deviation of the pitch.